Put simply, GDDR5 is fast approaching the limits of its specification. Bandwidth demands continue to grow, and for GDDR5 to keep up, it has had to increase its power consumption accordingly. However, each additional GB/s of bandwidth it squeezes out is beginning to consume too much power to be a sustainable or cost-effective option.
HBM effectively provides more bandwidth for less power, allowing GPU manufacturers to continue to increase bandwidth without running into a “power wall”. In hard numbers, that's over 35GB/s of bandwidth per watt, compared to GDDR5's 10.66GB/s – a 3.5x improvement.